This invention relates to magnets and a method of producing magnets, and more particularly, to metal-metal matrix composite magnets and a method of producing the composite magnets.
Neodymium-iron-boron (Nd.sub.2 Fe.sub.14 B) and its modifications, such as Nd.sub.2 Fe.sub.14-x Co.sub.x B and Nd.sub.2-y Dy.sub.y Fe.sub.14 B, are the strongest permanent magnets now known. Typically, these magnets have a strength of up to about 35 MGOe and are useful in applications at temperatures up to 300.degree. C. These magnets are produced by compacting a metallic powder and sintering the particles at temperatures above 700.degree. C., if under pressure, but often above 1000.degree. C. These magnets are difficult and relatively expensive to fabricate. U.S. Pat. No. 4,597,938 describes a process for producing permanent magnet materials.
Polymer-bonded magnets, while not as strong as pure Nd.sub.2 Fe.sub.14 B magnets, can be relatively inexpensively produced. Typically, polymer-bonded magnets have a strength of up to about 8 MGOe and are useful up to a temperature of about 100.degree. C. These magnets are used in applications such as small motors and actuator motors. These magnets are produced by bonding a magnetic material such as Nd.sub.2 Fe.sub.14 B in a polymer matrix.
An alternative to pure Nd.sub.2 Fe.sub.14 B magnets and polymer-bonded magnets are metal-metal matrix composite magnets. Metal-metal matrix composite magnets, like polymer-bonded magnets, are less expensive and less complicated to produce than pure Nd.sub.2 Fe.sub.14 B magnets. One advantage of metal-metal matrix composite magnets over polymer-bonded magnets is temperature resistance. Polymer-bonded magnets are limited to service temperatures which will not exceed the limits of what the polymer can withstand. The temperature limit for polymer-bonded systems is either the softening point of the polymer or when oxygen diffusion becomes possible. Most polymers with sufficient fluidity to be formed with a heavy loading of solids cannot be used in air at above 150.degree. C. An epoxy polymer, for example, at 100.degree. C. allows oxygen permeation to the metal magnetic materials which begins to corrode and lose its magnetic properties.
In a metal-metal matrix composite magnet, the upper limit for service temperature is set by the magnetic alloy in the magnet. For Nd.sub.2 Fe.sub.14 B, the ultimate upper limit for service temperature is the Curie temperature (T.sub.c) at about 310.degree. C. With Nd.sub.2 Fe.sub.14 B, as with any permanent magnet, when it is heated to T.sub.c, all remnant magnetism is lost. While the crystalline anisotropy is retained, the domains seek their lowest energy alignment. In the absence of an externally applied magnetic field, this alignment invariably has equal numbers of dipoles directed in each crystallographically allowed direction. This results in no net magnetic moment. On a practical basis, the maximum operating temperature will be less. This occurs because both the magnetization and coercivity of the Nd.sub.2 Fe.sub.14 B have significant negative temperature factors. As a result, when such a magnet is in service, the unit containing it will start to lose power at a lower temperature, a temperature far below the Curie temperature.
Another advantage of metal-metal matrix composite magnets over polymer-bonded magnets is in maximum achievable power product. Polymer-bonded magnets are rarely able to achieve power products over 8 MGOe. In metal-metal matrix composite magnets a higher fraction of magnetic material, typically 90 vol%, is present in the metal bonded magnets. A polymer-bonded magnet loaded to the same volume fraction as a metal-metal matrix bonded magnet would be close to a 98 wt% magnetic material. A two weight percent bond phase in a polymer-bonded magnet is not likely to give very strong bonding.
Yet another advantage of a metal-metal matrix composite magnet is its corrosion resistance to organic solvents and moisture. For example, over a life span of 10 or 20 years, a permanent magnet motor can have many opportunities for exposure to materials such as lubricants, lubricant carriers, grease solvents, and paint solvents. Any of these materials have the potential to deteriorate the plastic in a polymer-bonded magnet which can lead to failure. On the other hand, none of these materials will have any effect on a metal-metal matrix composite magnet.
A metal-metal matrix composite magnet has better moisture resistance than a Nd.sub.2 Fe.sub.14 B sintered magnet because most of the outer surface of a metal-metal matrix composite magnet is, for example, either copper, cobalt, or nickel and none of these elements are oxidized by water. Thus, moisture alone will have little effect on the magnet. Any deterioration of a metal-metal matrix composite magnet will be comparable to, or less than what would occur with a polymer-bonded magnet.
Metal-metal matrix composite magnets, however, are susceptible to attack by mineral acid or other electrolytes. Any Nd.sub.2 Fe.sub.14 B magnet will be damaged by exposure to oxidizing acid such as HNO.sub.3 or H.sub.2 SO.sub.4. A polymer-bonded magnet will suffer the least amount of acidic corrosion because, once the metal in the top layer is dissolved, the rate of attack will drop sharply. A metal-metal matrix composite magnet, by its bimetallic nature, is more susceptible to electrolytic corrosion than a sintered magnet. It is recognized, however, that any magnet may be protected by coating the final fabricated magnet or part with a corrosion resistant layer.
There is a growing interest in the magnet industry in producing metal-metal matrix composite magnets, as an alternative to pure Nd.sub.2 Fe.sub.14 B magnets and polymer-bonded magnets. It is known, for example as disclosed in Japanese Patent No. 62-137809, to produce a metal matrix-bonded neodymium-iron-boron alloy magnet by mixing a metal powder such as copper, aluminum, zinc or lead powder as a bond phase with a fine powder of the alloy magnetic material. The metal/magnetic material powder mixture is compression molded and then sintered to form a magnet of a specified shape. In this known process a layer of metal (bond phase) is not chemically deposited on the surface of the magnetic material to produce the bond, but the process simply involves physically mixing a magnetically inert metal powder and a magnetic metal powder. The resulting mixed powder is then sintered. A disadvantage of the above known process is that a power product of less than 6 MGOe is obtained. With respect to use of low-melting (i.e. &lt;400.degree. C.) metals such as lead, a metal-metal matrix composite magnet fabricated with such low melting metal may suffer loss of physical strength during its fabrication or its use due to its low softening point which may be reached before its Curie temperature.
There are numerous other known processes in which a fully formed magnet is plated or coated with copper or other metal for corrosion prevention, but none of the processes use the plating or coating step to actually bond the magnet.
It is desired to provide a process for producing metal-metal matrix composite magnets including chemically depositing a metal (bond phase) onto the surface of a magnetic material.
It is further desired to carry out a process for preparing the composite magnets at low process sintering temperatures such as less than 400.degree. C. and low plating temperatures such as from about -10.degree. C. to 20.degree. C. Low operating temperatures mean less expensive equipment for a production facility.